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The inner continental shelf (0–50 m water depth) off Santa Catarina, southern Brazil was investigated using seismic stratigraphic methods to identify and interpret the major seismic stratigraphic units and to reconstruct the late Quaternary evolution of the inner shelf and coast. The stratigraphy comprises a bedrock basement overlain by a thick (> 30 m) unit that comprises, in the distal portion, laminated muds and sands, and in the coast-proximal zone, offlapping deltaic deposits, whose upper surface has been reworked to form alongshore-prograding barrier spits. Deposition of this unit is tentatively attributed to sea levels (− 20 m MSL) during the late Pleistocene (MIS 5a, or 5c). A normal fault in the muddy units with a vertical offset of 3.5 m indicates episodic tectonic activity. Cut into this unit is a profusion of incised valleys, associated with sea level fall ultimately to − 120 m at the last glacial maximum (LGM). The associated subaerial unconformity is only preserved close to shore, where it has escaped subsequent wave ravinement. This, coupled with the well-preserved incised valleys implies the underlying substrate to be weakly cohesive material (mud). The incised valleys display a variety of infilling sediments interpreted variously as basal fluvial sediments, estuarine basin infill muds and marginal bar or barrier-associated sands deposited during the postglacial marine transgression. During a sea level stillstand at ca. -50 m, a 3.5 km-wide planar surface was cut into the unconsolidated coastal plain sediments forming a terrace on which a thick sequence of sands accumulated. This unit is regionally developed and the acoustic signature implies it to comprise littoral sand including a large aeolianite component. The accumulation of sand on this mainland beach/dune system is attributed to establishment of a mature littoral system with strong longshore drift. The fact that it rests on an erosional surface that truncates incised valleys, suggests the unit to be Holocene in age. It is interpreted to have been deposited during a still stand in the Holocene and to have undergone aggradation and cementation until it was overstepped by sea-level rise. Its surface and the coastal plain to landward were then subject to wave ravinement and a thin (ca. 5–6 m) layer of modern marine sands was deposited on the eroded surface. Sea-level rise continued to the regional glacio-isostatic highstand of ca. 5.5 m, after which regressive strandplains developed in response to a renewed mature longshore drift system similar to that prevailing during the − 50 m stillstand of sea level. In the intervening period, rapid shoreline translation over the low gradient surface inhibited the development of the longshore transport system. The three shoreline systems identified show marked differences that relate to sea level change and the influence of the underlying topography. Shoreline 1 (Pleistocene highstand) developed by localised wave-reworking of delta deposits and involved a series of alongshore-prograding barrier spits on a shoreline of alternating high-relief bedrock and littoral sands in coastal re-entrants. Shoreline 2 (− 50 m) was a major accumulation of beach and dune sand deposited as a mainland-attached beach dune system on a low-lying coastal plain environment. Shoreline 3 (the modern coast) comprises a series of headland-embayment beaches, strandplains and transgressive dunes with a strong bedrock control and a well-developed longshore drift system delivering sediment northward.

In an era of global rising sea level, quantifying future shoreline behaviour is a key societal concern. The potential for dramatic shoreline change via overstepping or barrier disintegration on barrier-lagoon coasts as a result of future rapid sea level rise has been considered by various authors, but documented examples of barrier overstepping generally involve either coarse grain sizes or early diagenesis as beachrock. Where overstepping has been inferred, the overstepped barrier is seldom preserved. Using high resolution seismic data we describe the mid-Holocene overstepping of a sandy coastal barrier system and subsequent geomorphological changes that ultimately transformed it into a strandplain fronted by a muddy shoreface. This complete change in character is interpreted in the context of rapid sea level rise (tentatively linked to the 8.2 ka event) during which the former sandy shoreface-barrier was overstepped and decoupled from the contemporary shoreline, leaving the latter sand-starved. Upon overstepping, the wave-influenced shoreline was displaced rapidly to the landward margin of the former lagoon. Preservation of the overstepped uncohesive sandy barrier is attributed to the relatively wave-sheltered location, rapid sea level rise and rapid burial by shoreface sediments. This situation provides an insight into the conditions under which overstepping occurs, and thereby, the future response of barrier-lagoon systems to predicted rapid rates of future sea level rise.

The diagenetic history of calcareous fossils is required for their application as palaeoenvironmental indicators. In this study, cathodoluminescence-microscopy (CL microscopy) and back scatter electron image-energy dispersive X-ray spectroscopy (BSE-EDS microscopy) were applied to Pliocene rhodoliths from the Azores Archipelago (NE Atlantic) in order to gain additional insight regarding the trace element content distribution throughout the algae thalli, and to ascertain palaeoenvironmental interpretations. Two types of luminescence were obtained: (1) high and (2) low luminescence. Rhodoliths with high luminescence are related with high concentrations of Mn(2+) in seawater and low luminescence rhodoliths are related with low concentrations of Mn(2+) in seawater. When the rhodoliths were deposited at about 4.0-4.5 Ma, the shoreline configuration of Santa Maria Island was much different than today. The influence of volcanic activity due to the extrusion of lavas and associated products and/or the presence of active shallow-water hydrothermal vents, was reflected in the sea water chemistry, with penecontemporaneous palaeoshores of the island featuring a high sea water concentration of Mn(2+), which mirrored on the rhodolith Mn(2+) high concentration. By contrast, rhodoliths located about 2.8 and 2.9 km from the shore, in areas with low seawater Mn(2+) concentration, had low luminescence, reflecting the low Mn(2+) concentration in seawater. Rhodoliths chemical data and the geological history of the island proved to be congruent with the palaeogeographical reconstruction of Santa Maria Island at the time of the formation of the rhodoliths.

Massive fossil shell accumulations require particular conditions to be formed and may provide valuable insights into the sedimentary environments favouring such concentrations. Shallow-water shell beds appear to be particularly rare on reefless volcanic oceanic islands on account of narrow, steep and highly-energetic insular shelves where the potential for preservation is limited. The occurrence of an exceptional coquina (Pedra-que-pica) within the Miocene–Pliocene deposits of Santa Maria Island (Azores), therefore provides a rare opportunity to understand the conditions that led to the formation and preservation of a massive shell bed at mid-ocean insular setting. This study provides a detailed analysis regarding a 10–11-m-thick bivalve-dominated fossil assemblage exposed at Pedra-que-pica on Santa Maria Island in the Azores. Integration of taphonomical, palaeoecological and sedimentological observations are used to reconstruct the genesis of the coquina bed and related events, and to discuss why such exceptional sedimentary bodies are so rare on shelves around reefless volcanic oceanic islands. The sequence at Pedra-que-pica demonstrates a complex succession of sedimentary environments in response to the drowning of an existing coastline during a period of rapid sea-level rise. The Pedra-que-pica shell bed incorporates storm-related materials and possible debris falls that originated nearby in a shallow and highly productive carbonate factory. Deposition took place below fair-weather wave base, at around 50 m depth, as inferred from the overlying volcanic succession. The preservation of this coquina was favoured by deposition on a platform laterally protected by a rocky spur, combined with rapid burial by water-settled volcanic tuffs and subsequent volcanic effusive sequences. The recent exhumation of the deposit is the result of island uplift and subsequent erosion.

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system. New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped. The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region. A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.